Dry recovery tests of plutonium-uranium mixed oxide fuel pellets, 2

The spalling characteristics of simtered Pu-U mixed oxide pellets by means of air oxidation were experimentally studied together with the solubility of the spalls in HNO3. The specimens used were the pellets containing 28.5 w/o PuO2 and with dimensions of 4.5 mm diameter and 9 mm height. Oxidation temperature (350 deg C - 600 deg C) and sintering temperature (1500 deg C - 1700 deg C) were chosen as major parameters. The physical properties of spalls and pulverized fragments were examined by X-ray diffraction method. For dissolution test, the sieved spalls with diameter smaller than 500 μm were used, and dissolved in boiling nitric acid (HNO3/H2O volume ratio = 1). Pu content was determined by the potentiometric titration. The effect of oxidation temperature on the spalling was examined at the sintering temperature of 1650 deg C, and good pulverization was observed at oxidation temperature between 350 deg C and 500 deg C. The effect of sintering temperature was also examined at oxidation temperature of 400 deg C and 500 deg C, and it was found that the pellets sintered at 1700 deg C were not able to be broken up at 500 deg C, but fractured at 400 deg C, and that the spalling occurred generally more effectively at 400 deg C than 500 deg C. The solubility test indicated that the spalls prepared at 450 deg C were most easily dissolved. It was also observed that a higher sintering temperature (1650 deg C - 1700 deg C) is necessary to attain high degree of the solubility. The significant effect of O/M ratio of the solubility was not observed under the present dissolving conditions. (Aoki, K.)